International patent application WO 2006/018749 discloses a near-field optical scanning device for scanning a record carrier. In a near-field optical scanning device a radiation beam is transmitted across a gap between an exit face of an objective system and an outer face of the record carrier using evanescent coupling of the radiation. A gap servo keeps the size of the gap during scanning at a distance of approximately 25 nm. A tilt servo in the device controls the tilt angle between the objective system and the record carrier such that it is kept within a relatively tight tilt tolerance. This tolerance may be as small as 1.2 mrad. The tilt signal is determined from variations in an efficiency of the evanescent coupling across the exit face of the objective system. The position of the focus with respect to the exit face can be varied by varying the position of a lens in a telescopic system arranged in the optical path of the radiation beam.
A disadvantage of this known scanning device is that no signal is available representing the position of the focus along the optical axis.
It is an object of the invention to provide an optical scanning device in which such a signal is available.